The present invention relates to an inspecting method and apparatus for a photomask pattern applied to the fabrication of a semiconductor device such as a large semiconductor integrated circuit.
A photomask pattern generally consists of a plurality of patterns, so called "unit patterns", each having the same shape and size for efficient fabrication of a plurality of semiconductor devices on a semiconductor slice by a printing process. The inspecting object of the present invention is such a photomask pattern consisting of a plurality of unit patterns.
A pattern inspection can be classified into two categories: a database comparing method, and a pattern comparing method. In a database comparing method, a unit pattern is compared with its design data, while in a pattern comparing method one unit pattern is compared with another unit pattern having the same shape and size.
Recently, semiconductor devices have become so complex with increases in packing density, that the database comparing method requires a lot of inspecting time and covers a lot of design data. Therefore, the pattern comparing method is preferable to the database comparing method for inspection of a complex photomask pattern.
The pattern comparing method can be also classified into two categories: an analog comparing method, and a digital comparing method. The digital comparing method is essentially preferable with respect to its accuracy, and is becoming popular because recently the digital technology has been advanced and its cost has lowered.
The digital pattern comparing method is performed as follows: (1) a pair of parts, assumed to have the same shape and size, in two unit patterns are selected by two respective optical systems; (2) the two optical images of the parts obtained by the respective optical systems are converted into two groups of analog video signals by respective image sensors in the optical systems; (3) the two groups of analog video signals are respectively digitized into two groups of digital video signals; (4) the two groups of digital video signals are stored as two groups of pattern data in two respective image memories, each having a memory matrix in which memory elements are arranged in correspondence to the arrangement of the picture elements in each optical image with the resolution required for the inspection; and (5) the two groups of pattern data are compared to each other.
The photomask is mounted on a stage to be inspected. The selection of the two unit patterns to be compared can be made by repeatedly shifting the stage in steps and the two optical images can be simultaneously obtained by scanning the stage.
The digital comparing method has the following advantage in comparison with the analog comparing method, the influence of electric noise caused by shifting or scanning the stage can be avoided. However, the digital comparing method also has a disadvantage in that it requires a lot of time for data processing; it takes as much as several times as long as with analog comparing method.
To improve this disadvantage, new technology for the digital comparing method called a "vector comparing method" has been developed and actually used. For example, the method is used in a photomask inspecting apparatus called "KLA 100 or 200 series" which is manufactured by the "KLA Corporation" in the U.S.A. The vector comparing method is derived from the concept that the difference between the data arrangements in two groups of pattern data can be treated as a comparing subject with the application of a vector means. Therefore, in the vector comparing method, it is not necessary to compare the pattern datum in every element of the image memory matrix, thus the data processing time can be shortened.
The vector comparing method is considered to be a very excellent method because of its high accuracy, high sensitivity, and short processing time, however, it has been found that there is a problem in that some kinds of defects in the pattern can not be detected. The present invention intends to solve this problem. Before disclosing the present invention, the prior art vector comparing method and its problems will be discussed.
FIG. 1 is a block diagram of an apparatus for vector comparing inspection of the prior art. In the figure, a photomask 1 is mounted on a stage 100 and light 3 is irradiated from beneath the photomask 1 so that two parts, one in each of two unit patterns, can be detected, one by a left optical system 201 and the other by a right optical system 202. The left and right optical systems 201 and 202 respectively consist of lens systems 211 and 212 and image sensors 41 and 42. Each of the image sensors 41 and 42 consists of linear arrayed sensor elements and each element corresponds to a picture element of the unit pattern which provides sufficient the resolution to meet the requirements of the inspection. Each group of video signals from the image sensor 41 and 42 is produced by scanning the stage 100 in a direction perpendicular to the arrayed direction of the image sensor. The distance between the optical system 201 and 202 is adjusted so that the optical images of corresponding parts in each of two unit patterns can be compared. The sequential comparison between corresponding portions of two unit patterns can be made by repeatedly by shifting the stage 100 in steps. The above scan and shift of the stage 100 are controlled by a controller 11. The two groups of analog video signals from the image sensors 41 and 42 are sent to an amplitude distributor 5 in which the two groups of analog video signals are respectively converted into two groups of digital video signals so that the amplitude of each analog video signal is distributed into three levels such as white (W), gray (G), and black (B) corresponding to the amplitude being low, medium, and high. In the three levels, the level G is provided to protect the occurrence of a false error. For example, when the amplitude of the analog signal is in a middle region, if there was no G level, the digital video signal would have to be either B or W; this would product a false error. The level G provides a margin to prevent a false error occurrence. The two groups of digital video signals are stored as pattern data in a first left memory 12L and a first right memory 12R by left and right write-in units 71 and 72 respectively, under the control of the controller 11.
When a photomask pattern is constructed with very high packing density, a minimum area as small as 0.5 (.mu.m).times.0.5 (.mu.m) is required for the inspection resolution of the photomask pattern. If the linear arrayed image sesor consists of as much as 512 sensing elements and the stage 1 scans a distance corresponding to 512 sensing elements, the optical image created is formed by 512.times.512 elements at the image plane of the optical system. This is equivalent to the size 256 (.mu.m).times.256 (.mu.m) on the photomask 1, and this is the area of a comparing part in the unit pattern.
To store the above pattern data, each memory matrix in first left and the right memory 12L and 12R has 512.times.512 matrix elements. However, a quantity of pattern data as large as 512.times.512 matrix elements is too large to be inspected simultaneously with high accuracy, so that the pattern data of 512.times.512 matrix elements in each of the first memories 12L and 12R are separated to a set of 4.times.4 matrix elements and stored respectively into a second left memory 13L and a second right memory 13R. The separation and memorization are respective performed by a left separator 81 and a right separator 82. The separated data become the subject of the vector comparing inspection and this separation and memorization are sequentially advanced every time the inspection of the separated data is completed.
One of the signals W, G, and B exists as a pattern datum in each matrix element of each second memory 13L or 13R; therefore a vector can be applied to the boundary between the neighboring elements. In the prior art, the vector is provided in four directions; left, right, up, and down with equal magnitude, and the direction is determined by the following definition: when the datum B and W are adjcently arranged along longitude or latitude, the vector direction is defined from B to W; when the datum G is surrounded by the data W in all four primary directions, four vectors from G to W are provided along longitude and latitude; when the datum G is surrounded by the data B in all four primary directions, four vectors from B to G are provided along longitude and latitude; and when the data are arranged like B-G-W in longitude or latitude, G is changed to B so that only one vector from the changed B to W is provided. Thus, in the prior art, a vector can be provided along longitude and latitude, but can not be provided along a diagonal.
These vectors are generated by a left vector generator 91 and a right vector generator 92, and the vector data are respectively fed to a left vector memory 14L and a right vector memory 14R in a vector memories 14. The vector data in the vector memory 14L and 14R are compared by a vector comparator 10 in every vector direction. For example, if there is a right directional vector in the left memory and no right directional vector in the right memory, the comparator 10 outputs the information that there is a defect; if there are two right directional vectors in the left memory and a right directional vector in the right memory, the comparator 10 outputs the information that there is no defect. Thus, the comparator 10 does not care about the total number of respective directional vectors in each memory, the comparator 10 only compares whether there are vectors having the same direction in the left and the right memory.
FIG. 2 is a flow chart for the prior art inspecting apparatus illustrated in FIG. 1. Reference numerals in FIG. 2 are the same as the block numerals in FIG. 1 to indicate where the function of the blocks exist. In addition, a stage coordinate memory 300 and an inspection output device 400 are included in FIG. 2. When the vector comparator 10 produces a "NO" signal, this means there is no defect, and the signal is fed back to the separators 81 and 82 to advance the inspection of the next separated pattern; when the vector comparator 10 produces a "YES" signal, this means there is a defect and the stage coordinate memory 300 records the stage position at which there is a defect. After the recording the position, the stage coordinate memory 300 advances the separator 81 and 82 to inspect the next separated pattern. When the inspection of the whole photomask pattern is finished, a human inspector can check the inspection results by observing the stored data in the stage coordinate memory 300 on the inspection output device 400 such as a cathode-ray tube or a sheet of printed paper.
FIG. 3 illustrates how to perform the prior art vector comparing inspection, showing an example of a defect on the photomask pattern. In FIG. 3, FIGS. 3(a-1) and 3(b-1) depict the optical images, i.e., the parts of the photomask pattern to be compared to each other. FIG. 3(a-1) depicts an optical image corresponding to the separated data in the left second memory 13L and FIG. 3(b-1) corresponds to that in the right second memory 13R. The parts of the oblique dotted lines located outside of the square frame in each figure indicate how the optical image continues in the photomask pattern. A pattern 15L in FIG. 3(a-1) has a defective part 20 and a pattern 15R in FIG. 3(b-1) is assumed to be normal. A data matrix 16L in FIG. 3(a-3) represents the separated data in the left second memory 13L, and a data matrix 16R in FIG. 3(b-2) represents the data in the right second memory 13R. Vector patterns 17L and 17R in FIGS. 3(a-3) and 3(b-3) respectively depict the vector pattern in the left vector memory 14L and the right vector memory 14R. Comparing the vector directions in FIGS. 3(a-3) and 3(b-3), the following results can be obtained: a right vector exists in both memories, a left vector does not exist in either memory, an up vector exists only in the left memory, and a down vector also exists only in the left memory. Therefore, the comparator 10 in FIG. 1 outputs the information that there is a defect in the photomask. Thus, defective part 20 in the separated pattern 15L can be detected by the vector comparing inspection.
The digital pattern datum at the defective part 20 was B in the above example. However, even if the pattern datum is G; that is, defect is as small as the half of the part (matrix element) 20, the same results can be obtained, because the data including the defective part 20 are arranged in B-G-W in latitudinal direction and this arrangement is changed to B-B-W in the vector generator 91. Thus, the vector comparing inspection is effective to detect a very small defect, and further, importantly, the inspection can be performed very quickly because the inspection can be done just by comparing the directions of respective vector data. However, it has been found that this inspection has a problem in that some pattern errors cannot to be detected even though the detected signal has enough amplitude like a datum B when it should be W or G.
FIGS. 4 and 5 illustrate the problems in the vector comparing inspection of the prior art, showing examples of indetectable defects. FIG. 4 shows a defect in a slant pattern. FIG. 5 shows square holes in respective separated patterns; corresponding sides of the square holes are parallel to each other and the shapes are the same, but the sizes are different.
In FIG. 4, the situation of each figure is the same as that in FIG. 3. A separated pattern 15L in FIG. 4(a-1) has a defective part 20 and a separated pattern 15R in FIG. 4(b-1) normal. As the pattern appears at a slant in the matrix frame, there are G pattern data at the slant edge of the pattern. So, the data matrices in the separated memories 13L and 13R are as indicated by data matrices 16L and 16R in FIGS. 4(a-2) and 4(b-2), and the vector patterns in the vector memories 14L and 14R are generated as in FIGS. 4(a-3) and 4(b-3). A comparison between the vector pattern 17L and 17R, finds the same quantity of the vectors in every direction. This indicates there was no defect on the photomask pattern.
In FIG. 5, the separated pattern data are as indicated in FIGS. 5(a-2) and 5(b-2). Thus, respective vector patterns are generated as in FIGS. 5(a-3) and 5(b-3). This result also indicates there was no defect, because the vectors in the left and the right memory are equal in every direction.
The undetected defects in FIGS. 4 and 5 often appear in an inspection of the photomask pattern, for example, the problem of the square holes occurs in the inspection of a contact hole in a large scale integrated circuit, and it is very important for the vector comparing methods to detect such defects if it is to be used with confidence.